This invention relates to fluid-gauging probes and their manufacture.
Fuel and other fluid levels in, for example, aircraft may be measured by means of a capacitance probe comprising an outer tubular electrode and an inner coaxial electrode. The probe is mounted to extend vertically in the fuel tank such that fuel fills the outer electrode to the same height as fuel in the tank. As the fuel level in the tank changes, therefore, a corresponding change in the level of fuel in the outer electrode is produced which causes a change in the value of capacitance between the two electrodes. By measuring the capacitance, the fuel level can be determined.
In many applications, the tank in which the probe is mounted does not have the same sectional area or profile at all heights, so that the same change in volume of fuel will not produce an identical change in depth at different heights. This, therefore, would cause the output of a conventional probe to be non-linear with volume. In order to compensate for this, the probe is commonly characterized in some way so that its output changes uniformly with volume, rather than with height. The usual way of characterizing such a probe is by varying the diameter of the inner electrode along its length. In this way, the gap between the inner electrode and outer electrode varies so that the change in capacitance as the gap is filled with fuel correspondingly varies.
The inner electrode can be constructed in different ways. The electrode could be machined with the required profile from a solid rod of material. This, however, is an expensive process and results in a heavy electrode which puts a correspondingly greater stress on its mounting when subject to vibration and shock. A thinner electrode can be produced by electro-depositing a metal coating onto a mandrel preformed to the required shape, the mandrel subsequently being dissolved or melted away to leave the outer metal shell. This electro-deposition process requires an expensive manufacturing plant which is not justified for only making low numbers of probes. The process itself is also expensive because of the need to make a mandrel for every probe. Another form of electrode is made up of discrete lengths of tubing of different diameters joined together by means of coupling pieces and rivets. This form of probe also has disadvantages, however, in that it can be difficult to establish a good electrical continuity between the different sections of tubing. This can be aggravated in time if the coupling pieces or tubing becomes corroded. Vibration and other forces on the probe can also produce high stresses on the joints and tend to loosen them. The rivets and the holes through the rivets cause electrical non-uniformity in the region of the joints and can affect the probe characteristic for fuel levels in these regions.